1967 C20 3/4 ton rear end gear help

[eastcoastcivil]

I have a 67 C20 with 4.10 gears in the rear end and I want to change them to something a little more highway friendly. Are the gears for a 1/2 ton the same as for a 3/4 ton? (I am guessing no) Can I use gears out of any other year truck? How hard is it to swap these gears in a 3/4 ton? Thanks

[pumpkin]

Stock diff on that is most likely an eaton. your only gear choices that are redily available are 4.10 4.56 and 5.13 . so if you want to lower your rpms, you need a different diff, taller tires or an overdrive trans.

[eastcoastcivil]

If thats the case I may just put a TH400 in it and leave the rear end gears alone...use it as a cruiser. I have 31" tires on it now. Thanks for the info.

[Longhorn Man]

in case you aren't aware, a TH400 will do nothing to lower your cruise RPM, but it is a very strong trans.

[eastcoastcivil]

Yeah I know a TH400 is still a 3 speed, but it will be less work to drive than the 3 on the tree. Thanks for the help.

[eastcoastcivil]

Does a 74-87 3/4 ton rear end have faster rear end gears? Would it bolt in, and would there be any mods? And by bolt in I mean the whole rear end.

[Longhorn Man]

do you have coils in back, or leafs?

[eastcoastcivil]

Coils springs.

[Longhorn Man]

no direct swaps then, not for a 14 bolt 73 & up rear.
You can look for a dana 8 lug rear out of a 68 - 72 big block truck/burb. It would have the 3.54 gear. That's the best upi'll do for direct bolt in aplications. If you locate one, try and get the rear driveshaft, it'll make your swap a lot easier, just slide it on your drive shaft and go)

[capev86]

i love the 3 on the tree saginaw in my 72 burb 3/4 ton 2wd and corp/eaton rear. unfortunately it has the 4.57 gear, not the 4.10....the engine is racing on the highway (so i generally keep it under 50mph). a 4.10 on a 3/4 ton is like a 3.73 on a 1/2 ton.....good compromise between power and economy, especially since the heavier trucks run taller tires. get an overdrive unit. i plan to keep my saginaw because it is original, works good, is convenient and out of the way like an auto shifter yet still allows me to have a stick. plus, it fools soo many people. you need to find the OD unit for a 65-70 full synchro saginaw that came in some 1/2 ton trucks. it should be rugged enough as long as you shift out of OD for towing like with any other overdrive. earlier units are not right. the problem with my truck is it has a 4.57 which is just too low unless all i did was pull trailers up hill at low speeds. i figure the stock 307 will do just fine with a 4.10 swap and the overdrive will make it truly highway friendly. i have a 700R4 out of an 84 burb. is it is currently attached to a 406sbc that is going in my truck some day. if my rig had been an auto, i would have gotten an adapter crossmember and swapped it in, but i like the stick so it is staying.....be different from the trend.

[Longhorn Man]

i disagree. I would never install that real weak very outdated overdrive in a 3/4 ton truck.
A 3.54 rear gear will drop the RPMs about the same as that overdrive unit, and it considerably stronger.
Also, since the OP has kinda indicated either being new to the hobby, or just not that great at fabbing, a rear axle swap is very simple and straight forward, unlike trying to locate something that is very hard to find, verify that it even works, then wire it up to where it will work.
While that is my opinion, it is a good thing to throw the ideas out.
There is also the idea of going to an overdrive tranny, be it modifying a later model factory unit, or an add on aux transmission like US gear or gear vendors.

[eastcoastcivil]

Your right....this guy is just getting into the hobby and is learning, but still needs direction. I appreciate all the help and I think that swapping the rear end is far easier for me at this stage in the game. lol. Thanks guys.

[capev86]

i am a paying member now, so.....

if you change over to a dana or whatever, i request that you put that rear axle on hold for me if:

1) it is indeed a corp/eaton with 4.10 (not a 4.57)
2) it is in good shape w/ no noise
3) you are willing to let it go for a low price to help a guy out

i can can come pick it up easily as new brunswick and maine are next door neighbors. it should fit in the back of my sable wagon (gets much better gas mileage at much faster speeds than my burb w/ a 4.57 and no OD). i will likely just swap the third member and run my axle and keep that housing as a spare just to get it out of your way unless someone else needs the housing.

much appreciated if this is ok with you!

[eastcoastcivil]

I will take some pics of what I got in the rear now so someone can Identify it for me. Is there anything specific I should take a pic of?

[Gumby'sC20]

I too would also like to know what 3.73 rear end from what generation trucks would work...and which ones are 8 lug...if any?

[capev86]

if you want a 3.73....just swap in a 14 bolt from a later gm truck. 2000+ 14 bolts have a 11.5 ring gear (vs. 10.5 on the older generation 14FF) and feature disc brakes that are much better than aftermarket kits. i will probably adapt the brakes to my burb some day, but i really like the looks of that corp/eaton 10 bolt which is the precursor to the 14BFF. also, a 14 bolt swap will require grafting over the trailing arm perches and track bar mount. i believe you can get the perches for 3" diameter axle tubes, but they may have to be modified to fit the larger diameter axles on the hd trucks (haven't measured mine).

3.73 only came in the 12 bolt rear on the 1/2 ton models in this era truck. with a 3/4 ton rig with coil springs and trailing arms, the only factory bolt in choices are a corp/eaton with a 4.10 or a dana 60 with 3.54 gears. if you have leaf springs, then a 14 bolt swap is a bit easier, but you will still have to move the perches in a bit because the 67-72 trucks have a slightly narrower frame. advantage is you get a slightly wider track width (front track is often wider from the factory). i imagine the 2000+ trucks have an even wider track. remember that dual wheel axles are narrower to make room for the extra pair of tires.